David Harewood: 'I was terrified when I got the part of playing Mandela'

In a BBC film, David Harewood has just had the best role of his career. Yet, as he tells Ian Burrell, he must leave Britain behind if he is to fulfil his acting potential

For David Harewood it should really be a high point, the crowning achievement of a career in which he has enjoyed success on stage, television and in film. Instead, this British actor is a melancholic figure, determined to quit this country at the end of this month to seek a more fulfilling fame abroad.

There's good reason for this: Harewood's sadness is born of grief. What should have been a triumphant year for him, one in which he got to play Martin Luther King on the London stage, and Nelson Mandela in a 90-minute film for the BBC, has culminated with the sudden death of his closest friend.

"It's as if someone has just basically turned the light off in my life. I haven't worked since. It has just been the flip of all the success. I have become introspective, I haven't had any money, it has just been a complete nightmare," he says. For 35 years, since they sat at adjoining desks at their Birmingham school, Lui had been Harewood's soul-mate and sounding board. He died from a complication in what should have been routine knee surgery.

"He was 43, the same age as me. He was going to be the best man at my wedding," says Harewood. "His wife phoned me up about a week after the funeral and told me she was pregnant with their third. It's made me think," he adds, tearfully. "Sometimes it doesn't always work out and you've got to make it happen. I'm going to go off to Hollywood and see if I can make something happen for myself over there. He always said I should go."

The actor had come to the Charlotte Street Hotel in central London not to discuss this tragedy but to talk about the powerful BBC biopic Mrs Mandela, shot in Johannesburg, in which he plays the greatest living African opposite Sophie Okonedo's Winnie.

Harewood is known to British theatre-goers for his Shakespearean roles, most notably an acclaimed performance as Othello in a Sam Mendes production for the Royal National Theatre. Cinema audiences may remember him as the sadistic rebel leader Captain Poison in Blood Diamond, and he has appeared in countless weekend TV dramas, including Dr Who and Robin Hood.

But he has never prepared for a role as he prepared for Mandela. "When I got it I was terrified, to be honest with you. I wanted to make sure I treated him with respect. Normally I like to be a clown on set, but this was the first time I really decided not to do that." He read Mandela's autobiography and Anthony Sampson's biography, borrowed documentary films from the library and watched others online.

"I started with the myth, an icon, but the research led me to the man. Once I got the man I felt it was just a character I had to play and I didn't feel burdened by any sort of responsibility," he says. "He felt tremendously guilty about Winnie. Once I had found those colours I felt able to play him. I think he's tremendously vulnerable in the film."

During filming in Johannesburg, he stayed in a hotel room that looked down on Mandela's statue in Mandela Square, and yet he had to remind himself that Okonedo had the starring role in the film, which shows how she was tortured by the authorities during years of enforced separation from her husband, whose memory she fought to preserve. Some white South Africans among the film crew had been unaware of the mistreatment of Mrs Mandela – who is shown leaving a room shortly before her bodyguards administer a fatal beating to teenage African National Congress activist Stompie Moeketsi.

Though he had to play Mandela "in snippets", Harewood is proud of his achievement, particularly his mimicry of the South African leader's voice. "Even the sound guy would say, 'Close your eyes and it's as if he's in the room' – and these are South Africans so I was incredibly proud of that.

"We did a scene where I had to speak on a balcony to about 70 people, and when I started you could see them really quite stunned. When they had to applaud they were absolutely in raptures, it was extraordinary to watch them break out into these spontaneous freedom songs."

It has been hard to come back from such a role and look for work in a "very tough" environment in which both ITV and Channel 4 are struggling to find money for drama. "It's pretty hard to stand in the queue auditioning to play a gynaecologist on Holby City when you've just played Mandela. You think, 'actually I want to challenge myself'." Having hired an American agent he has become aware of the opportunities that do exist for black actors: "I have had more scripts from America in three months than I had in England in three years. Specifically black parts, but with real authority."

Harewood was the class clown at school, only turning to acting at the suggestion of an English teacher who called him at home to ask what he was going to do with his life. To Harewood's own surprise he was accepted first by the National Youth Theatre and then by RADA. At each stage of his 27-year career, he had shared the adventure with Lui. "Acting, to be honest, has always been a bit of a giggle; if I got a part I'd ring him up," he says.

So he has thought of Lui's advice, and of the path taken by black British acting friends such as Lennie James, David Oyelowo, Chiwetel Ejiofor and Idris Elba, and has decided to relocate to the US. "We are a tight community because we are all going after the same jobs," says Harewood. "They're all living in LA. I'm probably the only one of my peers still here. It's time to go."

Mrs Mandela is showing on BBC Four at 9pm tonight